Polyolefin porous films have been used widely as a separator in batteries, capacitors, and the like because of their excellent electrical insulation properties and ion permeability. Particularly in recent years, lithium ion secondary batteries of high power density and high capacity density have been used as a power source of mobile phones required to incorporate an increasing number of functions and be light in weight. Polyolefin porous films are mainly used as a separator for such batteries.
Although lithium ion secondary batteries have a high power density and capacity density, an organic solvent used for their electrolytic solution is decomposed by heat generated due to abnormal circumstances such as short circuit and overcharge and it may lead to ignition at worst. Lithium ion secondary batteries are equipped with some safety functions in order to prevent such circumstances. One of them is a shutdown function of a separator. The shutdown function means a function of closing micropores of the separator by heat melting or the like to suppress ion conduction in the electrolytic solution and stop the progress of an electrochemical reaction when abnormal heat generation of batteries occurs. Separators having a lower shutdown temperature are regarded to have higher safety. Polyethylenes are used as a component of the separator partly because they have an adequate shutdown temperature. In batteries having a high energy, however, a temperature in them continues to increase even if the progress of the electrochemical reaction is terminated by shutdown, which may result in film breakage due to heat shrinkage of the separator and occurrence of short circuit between two electrodes.
With a view to overcoming such a problem, there is proposed a method of forming a layer comprising an inorganic filler between a separator and an electrode (Patent Document 1). This method enables to prevent short circuit between two electrodes because the inorganic filler-containing layer exists as an insulation layer even if a temperature continues to increase and exceeds the shutdown temperature and as a result, the separator breaks.
Patent Document 1: Japanese Patent No. 3756815